Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $34.50   10.497%   $38.1215   $38.12   $38.10   $38.10  
  Purchase #2 $27.31   3.247%   $28.1968   $28.20   $28.20   $28.20  
  Purchase #3 $123.69   11.551%   $137.9774   $137.98   $138.00   $138.00  
  Purchase #4 $124.83   8.328%   $135.2258   $135.23   $135.25   $135.20  
  Purchase #5 $22.34   6.440%   $23.7787   $23.78   $23.80   $23.80  
  Purchase #6 $210.74   7.124%   $225.7531   $225.75   $225.75   $225.80  
  Purchase #7 $74.30   7.401%   $79.7989   $79.80   $79.80   $79.80  
  7 purchase totals = $617.71       $668.8522   $668.86   $668.90   $668.90